1. Field of the Invention
This invention relates to the field of nuclear magnetic resonance (NMR) well logging apparatus and methods. More specifically, the invention is related to methods for processing NMR signals to determine fluid and gas properties, relative and total amounts of fluids, and the ability to produce these fluids in fluid-bearing earth formations penetrated by wellbores. Additionally, the pore size distribution of the porous rock containing these fluids is estimated.
2. Description of the Related Art
Downhole characterization techniques are of considerable value for geophysical exploration. For example, characterization of parameters associated with fluid-bearing earth formations provides insight into quantifying hydrocarbons and water present in the formation. A number of technologies are applied to determine the parameters. These technologies include nuclear magnetic resonance (NMR) imaging. An apparent transverse relaxation time constant distribution may be used to analyze water, oil and gas zones.
When a reservoir rock, clastic or carbonate, is completely filled with water, a distribution of apparent transverse relaxation time constants (T2,app) will reflect the pore size distribution of the rock due to interaction with the rock surface. This interaction is referred to as surface transverse relaxation and is measured by a surface transverse relaxation time constant (T2,surf). Often rock samples are saturated with water in a laboratory to determine the value of T2,app that separates the irreducible water in the reservoir from the water that can be produced when the rock is in-situ under normal production conditions. That value of T2,app may be referred to as irreducible water cut-off (or “COBVI”). FIG. 1 is a graph of incremental porosity versus T2,app. Referring to FIG. 1, the COBVI value of T2,app is depicted along with a value of T2,app (“COCBW”) that separates clay bound water from the irreducible water.
However, in reservoirs determining COBVI is complicated by the presence of hydrocarbon. Both oil and gas respond differently than water to the NMR measurements. The oil and gas respond differently because, unlike water, the oil and gas generally do not exhibit surface transverse relaxation. It is presumed that the oil and gas do not contact the water-wet rock. In the case of oil and gas containing reservoirs, the hydrocarbon NMR signals interfere with the T2,app of water and, therefore, interfere with determining pore size distribution. The interference is an acute problem in carbonate rocks where the COBVI is highly variable and the T2,app signal from oil and gas often occurs within the T2,app signal for the irreducible water.
Therefore, what are needed are techniques for removing the oil and gas NMR signals from the distribution of apparent transverse relaxation time constants.